Heretofore, in order to heighten fuel economy (gasoline mileage) of a vehicle provided with an automatic transmission, it is known a line pressure control device for an automatic transmission capable of switching line pressure, which is established in a hydraulic control device for the automatic transmission, between two steps including low line pressure and high line pressure (for example, see Japanese Examined Patent Application Publication No. 6-15900 (hereinafter, referred to as “Patent Literature 1”) and Japanese Patent Application Publication No. 2002-89680 (hereinafter, referred to as “Patent Literature 2”)).
In the line pressure control device for the automatic transmission disclosed in Patent Literature 1, line pressure during shifting of gears is controlled in accordance with temperature of hydraulic oil, detected by an oil temperature sensor, for the automatic transmission. More specifically, it is first determined whether line pressure required during shifting of gears is in a low temperature region, a high temperature region or a shifting region between the low temperature region and high temperature region. Then, in the case where the line pressure required during shifting of gears is particularly in the shifting region, it is processed so that the line pressure to be controlled is caused to change gradually (or linearly) between a line pressure value for low temperature and a line pressure value for high temperature. Since this line pressure control device is configured in this manner, the line pressure is changed gradually. This makes it possible to achieve an effect that gear shift shock can be reduced by controlling line pressure without occurrence of a sudden change in the line pressure.
Further, in the line pressure control device for the automatic transmission disclosed in Patent Literature 2, line pressure is controlled so that, when a throttle valve opening degree is lowered in a stepwise manner, it is determined that line pressure is switched from high pressure to low pressure and start of this switching is delayed by a predetermined period of time since the determination has been made. This line pressure control device is configured in this manner, whereby it is possible to prevent a frictional engagement element from slipping due to a transiently changing state of torque when engine torque is reduced. For that reason, effects that it is possible to avoid gear shift shock and to prevent duration of life of a friction member of the frictional engagement element from being lowered are achieved.
Moreover, in the control of the line pressure, for example, as shown in FIG. 4, it is known that a low line pressure region and a high line pressure region are classified using mapped data on the basis of a throttle valve opening degree of a vehicle (longitudinal axis, a ratio when an effective opening degree of a throttle is divided into eight) and vehicle speed (horizontal axis). In the case of using such mapped data, the low line pressure region is set so that a torque capacity of a clutch (frictional engagement element) does not fall below engine torque at an arbitrary throttle valve opening degree and vehicle speed.